Wednesday, 24 October 2012

Higgs: New Deal

The new round of Higgs data will be presented on the 15th of November at a conference in Kyoto, and on blogs a few days earlier. The amount of data will increase by about 2/3 compared to what was available last summer. This means the errors should naively drop by 30%, or a bit more in the likely case of some improvements in the analyses. Here's a short guide to the hottest Higgs questions that may be answered.

Will the γγ rate remain high? Last summer the Higgs boson showed up quite like predicted by the standard model. The most intriguing discrepancy was that both ATLAS and CMS saw too many Higgs decays to photon pairs, exceeding by 80% and 60% respectively the standard model expectation. Statistically speaking, the excess in both experiments is below 2 sigma, so at this point all the observed rates are in a decent agreement with the standard model. But that doesn't stop of us from dreaming and crossing our fingers. If the excess is a statistical fluke we would expect that the central value of the measured H→γγ rate will decrease, and that the significance of the excess will remain moderate. But if, purely hypothetically, the central value remains high and the significance of the excess grows then.... well, then it's gonna get hot.

Will the ττ rate remain low? Another puzzling piece of Higgs data from last summer was that CMS failed to see any excess in the H→τ+τ- channel, despite their expected sensitivity being close to the predicted standard model rate. In fact, they came close to excluding the 125 GeV standard model Higgs in that channel! This discrepancy carries less weight than the diphoton excess because it is reported by only one experiment (ATLAS did not update the ττ channel with 8 TeV data last summer) and because the strong limit seems to be driven by a large negative background fluctuation in one of the search categories. Nevertheless, it is conceivable that something interesting is cooking here. In 3 weeks both experiments should speak up with a clearer voice, and the statistics should be high enough to get a feeling what's going on.

Is the Vh → bb rate enhanced?The LHC has proven that Higgs couples to bosons: gluons, photons, W and Z, however it has not pinpointed the couplings to fermions yet (except indirectly, since the effective coupling to gluons is likely mediated by virtual top quarks). As mentioned above, no sign of Higgs decays to tau lepton pairs has been detected so far. Also, the LHC has not seen any clear signs of Higgs decays to b-quarks (even though it is probably the most frequent decay mode). On the other hand, the Tevatron experiments in their dying breath have reported a 3 sigma evidence for the h → bb decays, with the Higgs produced in association with the W or Z boson. The intriguing (or maybe suspicious) aspect of the Tevatron result was that the observed rate was twice that predicted by the standard model. In 3 weeks the sensitivity of the LHC in the b-bbar channel should exceed that of the Tevatron. It is unlikely that we'll get a clear evidence for h→bb decays then, but at least we should learn whether the Tevatron hints of enhanced Vh → bbcan be true.

Will they see h→Zγ? Another possible channel to observe the Higgs boson is via its decay to 1 photon and 1 Z boson, where Z subsequently decays to a pair of charged leptons. Much like in the well-studied h→ZZ→4l and h→γγ channel, the kinematics of the h→Zγ→γ2l decay can be cleanly reconstructed and offers a good Higgs mass resolution. The problem is the low rate: the Higgs decay to Zγ is even more rare than that to γγ, plus one needs to pay the penalty of the low branching fraction for the Z→l+l- decay. According to the estimates I'm aware of, the LHC is not yet sensitive to the h→Zγ produced with the standard model rate. However, if we assume it's new physics that's boosting the h→γγ rate, it is very likely that the h→Zγ rate is also boosted by a similar or a larger factor. Thus, it interesting to observe what limits can the LHC deliver in the h→Zγ channel, as they may provide non-trivial constraints on new physics.

Does Higgs have spin zero? Obviously, this question carries a similar potential for surprise as a football game between Brazil and Tonga. Indeed, spin-1 is disfavored on theoretical grounds (an on-shell spin-1 particle cannot decay to two photons), while a spin-2 particle cannot by itself ensure the consistency of electroweak symmetry breaking as the Higgs boson does. Besides, we already know the 125 GeV particle couples to the W and Z bosons, gluons and photons with roughly the strength of the standard model Higgs boson. It would be an incredible coincidence if a particle with another spin or parity than the Higgs would reproduce the event rates observed at the LHC, given the tensor structure of the couplings are completely different for other spins. Nevertheless, a clear experimental preference for spin-0 would be useful to satisfy some pedantic minds or some Nobel committees. In particular, one needs to demonstrate that the Higgs boson is produced isotropically (without a preferred direction) in the center-of-mass frame of the collision. With the present statistics it should already be possible to discriminate between spin-0 and alternative hypotheses.

So, keep your ear to the ground, the data are being unblinded as we speak, and the first numbers are already being bandied about in cafeterias and on facebook. Intriguingly, this blog post clearly hints there is a lot to rumor about in the new data ;-) Is it the high γγ rate? The low ττ rate? Something else? Well, there's still 3 weeks left and the numbers may shift a bit, so let's not spoil the fun just yet... In any case, if you have an experimentalist friend now it's the best time to invite her to a drink or to dances ;-)

Appears LHC has collected 1.7 times more data than last summer...unless I'm reading this online data chart from the LHC incorrectly? See link http://lhc-statistics.web.cern.ch/lhc-statistics/index.php?#

If 2011 3.5 TEV data is added into the 2012 4 TEV data, the increase in data for the upcoming Higgs analysis is 100% (18 fb + 5.3) / (6.6 + 5.3) relative to last July confirmation basis, assuming I'm using the correct data from the LHC stats link?

I'm hearing rumors that ATLAS will not be presenting updated 4l and gamma gamma data next week due to a discrepancy in the central value for the mass in the two channels. Makes one nervous about the long-term survival of the slight gamma gamma enhancement...care to monger?

Fascinating post! As a fellow science-lover, I thought you might be interested in theoretical physicist Sean Carroll’s new book about the Higgs boson, The Particle at the End of the Universe. I’m happy to share more info with you and send a review copy if you’re interested.

About Résonaances

Résonaances is a particle physics blog from Paris. It's about the latest news and gossips in particle physics and astrophysics. The posts are often spiced with sarcasm, irony, and a sick sense of humor. The goal is to make you laugh; if it makes you think too, that's entirely on your own responsibility...